ANS:C - introduces a compressive layer on the surface.

The high endurance limit of carburised machine parts is primarily due to the fact that carburisation introduces a compressive layer on the surface of the material. This compressive layer helps to counteract the tensile stresses experienced by the material during operation, thereby reducing the likelihood of fatigue failure. When a material is carburised, carbon atoms are diffused into the surface layer of the metal, typically steel. This process creates a hardened surface layer with higher carbon content, which is more resistant to wear and fatigue. Additionally, the diffusion of carbon atoms into the surface creates compressive stresses, which oppose the tensile stresses that cause fatigue failure. By introducing this compressive layer, carburisation effectively increases the fatigue strength and endurance limit of the material, making it more durable and long-lasting in service. This is especially beneficial for components subjected to cyclic loading or high-stress environments, such as gears, shafts, and bearings in machinery and automotive applications.